Clean drinking water supplies are considered a major public health milestone of our generation. However, waterborne diseases are still the third leading cause of death. The Word Health Organization (WHO) reported that diarrheal disease from unsafe water kills 1.5 million children each year and causes two billion cases of illness worldwide (WHO Diarrhoeal disease, fact sheet No 330). The problem is expected to be exacerbated by climate change (Gleick et al., 2010). A recent study shows that rich and well-developed countries with established municipal drinking water treatment and distribution systems are also vulnerable to waterborne pathogens. Reynolds and Gerba showed that from 1971 to 2002 there were 764 documented disease outbreaks related to drinking water in the United States of America resulting in roughly 0.6 million cases of illness and 79 fatalities (Reynolds et al., 2008 and Reynolds et al., 2011). They estimated that in the U.S.A. alone 26 million cases of infection and 13 million cases of illness each year are associated with unsafe municipal water caused by the inadequacy of treatment plants, contamination of water sources and intrusions into water distribution systems.
Drinking water distribution systems harbor rich microbial communities and the conventional method of using residual disinfectants is often ineffective in controlling the microbial growth (Servais et al., 1995, Kilb et al, 2003, Wingender et al, 2004 and Berry et al., 2006). Many problems in drinking water distribution systems are microbial in nature, from biofilm growth to water nitrification, corrosion, and persistence of pathogens (Camper et al., 2004, Emtiazi et al., 2004, Regan et al., 2003 and Beech et al., 2004). Although most of the microorganisms found in drinking water distribution systems belong to autochthonous aquatic microflora (Bagh et al., 2004), which are considered to be harmless, many studies have shown that drinking water biofilms can harbor opportunistic pathogens harmful to humans (Flemming et al., 2002, Szewzyk et al., 2000 and LeChavallier et al., 1996). Indeed, many waterborne pathogens are known to persist and reproduce in the drinking water distribution systems and are responsible for causing infections of gastrointestinal tract, skin and lymph nodes. Legionella pneumophila, Pseudomonas aeruginosa, Aeromonas sp and Mycobacterium sp are among the pathogens found in tap water in homes, public buildings and hospitals (Stojek et al., 2008, Armon et al., 1997, Moritz et al., 2010, Sen et al., 2004, Mazari-Hiriart et al., 2005, Kunimoto et al., 2003 and Dailloux et al., 2003). The situation is aggravated by the growing occurrence of antibiotic resistant genes in drinking water biofilms responsible for vancomycin-resistance (vanA) and beta-lactamase activities (Schwartz et al., 2003). Furthermore, many bacteria can transform into ultramicrocells (Silbaq et al., 2009) or enter into a viable but non-culturable state in response to environmental stress making surveillance and detection difficult (Oliver et al., 2005).
Microorganisms can colonize and form complex microbial ecosystems on all surfaces of drinking water distribution systems that are in contact with water (Vaerewijck et al., 2005). The age, construction and materials of the water distribution system affect biofilm formation and dynamics (Lautenschlager et al., 2010, Bolton et al., 2010 and Moritz et al., 2010). It has been shown that the diverse microbial community found in drinking water systems can increase microbial resistance to chemical disinfection. Studies have clearly demonstrated that maintaining a residual level of chemical disinfectant in drinking water distribution systems is ineffective in controlling microbial growth (Servais et al., 1995, Kilb et al, 2003, Wingender et al, 2004 and Berry et al., 2006). Higher chlorination combined with frequent flushing is shown to alleviate but not completely solve, the microbial problem (Besner et al., 2008). However, chlorination can produce unwanted byproducts such as chloromethane with its own health implications (Sohn et al., 2004). The same issue problem exists with ozone treatment in it produces byproducts, which byproducts as well as the ozone itself, have adverse health implications. (Galapate et al., 2001). UV disinfection at point-of-use has been proposed (Cristea et al., 2009), but at a significantly higher cost, and turbidity from gas bubbles can significantly reduce its effectiveness (Sommer et al., 2000). Also, cellular repair mechanisms limit its efficiency as shown by a study that reports Gram-negative Enterobacteriaceae (i.e., coliforms and enterococci) exhibit high rate of regrowth after UV disinfection (Sommer et al., 2000). A recent study also shows UV irradiation can induce competence in Legionella pneumophila allowing the bacteria to acquire and propagate foreign genes, contributing to its emergence as pathogen (Charpentier et al., 2011). Point-of-use water filters are reported to be effective in reducing Legionella pneumophila and Mycobacterium in water fixtures (Sheffer et al., 2005), but a more recent study shows that water filters are also vulnerable to microbial colonization (Chaidez et al., 2004). Heterotrophic plate count (HPC) bacteria, faecal coliforms, acid-fast organisms (Mycobacteria spp.) as well as opportunistic pathogens such as Aeromonas hydrophila, Plesiomonas shigelloides and Pseudomonas aeruginosa are reported to thrive in filtered water samples. The study concluded that many of the point-of-use filter devices may amplify the number of bacteria present in the tap water by promoting biofilm growth. Furthermore, current filtration technology cannot treat ultramicrocells in drinking water (Silbaq et al., 2009).
Pulsed electric field (PEF) treatment has been successfully used for water disinfection (Espino-Cortes et al., 2006, Uchida et al., 2008, Riedel et al., 2008 and Duda et al., 2011). The technology was first used for non-thermal sterilization pharmaceuticals (Goldberg et al., 2009) and food products (e.g., fruit juices, beer, milk and cream) (Rastogi et al., 2003 and Wan et al., 2009). In general, it was observed that Gram-negative bacteria (e.g., Escherichia coli or Pseudomonas aeruginosa) and yeast cells are much easier to kill than Gram-positive bacteria (e.g., Staphylococcus aureus or Enterococcus faecium) (Min et al., 2007). This higher resistance of Gram-positive bacteria to PEF is believed to be related to the cell wall composition of Gram-positive bacteria. PEF treatment can also sterilize bacterial spores and mold ascospores (Marquez et al., 1997 and Choi et al., 2008).
Inactivation of microorganisms by PEF relies on electroporation of the cellular membrane when an external electric field is applied (Sale et al., 1967 and Timoshkin et al., 2004). The accumulation of charges on the cell membrane eventually develops into a transmembrane potential that increases cell permeability and in severe cases lead to an irreversible breakdown of the cell. Electroporation is reported to occur at a transmembrane potential of 0.5 V and cell lysis near 1.5 V (Fox et al., 2005). Even sub-lethal injuries caused by PEF treatment is reported to render the microorganism nonviable within an hour of the treatment (Garcia et al., 2005). Reports also suggest PEF can decrease the activity of various enzymes including lipase, glucose oxidase, amylase, peroxidase and polyphenol oxidase (Ho et al., 1997), and could explain the observed change in the respiratory activity of PEF-treated bacteria (Podolska et al., 2009). Unlike thermal and UV treatment, PEF treatment does not induce competence in bacteria population (Riedel et al., 2008).
A number of PEF systems operating at high voltages have been used for microbial disinfections (U.S. Pat. Nos. 6,379,628, 6,083,544, 7,059,269, 6,746,613, and U.S. Patent Application Publication No. 2010/0112151). Although these devices show promising antimicrobial performance, they are unsuitable for point-of-use disinfection of drinking water from the tap due to the high voltage needed to operate effectively.